1. Field of the Invention
Preferential complexation of one organic compound in a mixture of related compounds is a known technique for resolving mixtures of closely related compounds. Of particular interest herein are methods for resolving mixtures of phenolic compounds, at least one of which is a nitrated phenolic, by preferential complexation of one of the phenolics.
2. Description of the Prior Art
Pure nitrophenols are desirable compounds for the preparation of other organic compounds usually via the corresponding amino phenols which can be obtained by hydrogenation. A nitrated phenolic of commercial importance is 4-nitro-m-cresol which is the precursor for a contact insecticide and acaricide, known generically as fenitrothion, i.e., O,O-dimethyl-O-(4-nitro-m-tolyl) phosphoro thioate. However, the nitration of phenolic compounds is not selective, usually leading to mixtures of ortho- and para- and 2,4-nitrated derivatives. The isolation and purification of individual nitrated phenolics is cumbersome, expensive and, possibly, dangerous by the use of conventional separation methods such as fractional distillation, which tends to cause decomposition and, possibly, explosions. Steam distillation, which has been used to separate ortho- from para-nitrated phenols, is energy intensive.
There are chemical processes known for separating closely-related organic compounds by methods other than, or in addition to, energy-intensive physical separation techniques such as fractional distillation or fractional crystallization. These chemical processes involve a step of preferential complexation of one component of a mixture of closely-boiling compounds over other components of the mixture. For example, U.S. Pat. No. 4,267,389 to Leston, describes treating a phenolic mixture comprising para-cresol, methylated phenols and ethylated phenols with an inorganic halide salt, such as calcium bromide, to remove para-cresol from the mixture. Removal of para-cresol from the mixture involves formation of a complex between para-cresol and calcium bromide, which complex forms preferentially over complexes between calcium bromide and other components of the phenolic mixture.
Mixtures of various alcohols may be resolved by treatment with a halide salt. For example, in Sharpless et al., J. Org. Chem., Vol. 40, No. 9, p.p. 1252-1257 (1975), there is reported a study of competition between pairs of mono-hydroxy alcohols and mono-hydroxy phenols for complex formation with a halide salt. This study finds that phenols as a class form poorer complexes than alcohols of comparable melting point, probably because the phenols are weaker bases than the comparable alcohols.
There remains need, therefore, for methods for resolution of mixtures of closely-related phenolics by chemical complexation methods, rather than by fractional crystallization or distillation.